[doc] Correct optimisation levels documentation for -fstore-merging

	* doc/invoke.texi (-fstore-merging): Correct default optimization
	levels at which it is enabled.
	(-O): Move -fstore-merging from list to...
	(-O2): ... Here.

From-SVN: r244893

gcc/ChangeLog

+2016-01-25  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+	* doc/invoke.texi (-fstore-merging): Correct default optimization
+	levels at which it is enabled.
+	(-O): Move -fstore-merging from list to...
+	(-O2): ... Here.
+
 2017-01-25  Richard Biener  <rguenther@suse.de>
 
 	PR debug/78363

gcc/doc/invoke.texi

@@ -7016,7 +7016,6 @@ compilation time.
 -fsplit-wide-types @gol
 -fssa-backprop @gol
 -fssa-phiopt @gol
--fstore-merging @gol
 -ftree-bit-ccp @gol
 -ftree-ccp @gol
 -ftree-ch @gol
@@ -7076,6 +7075,7 @@ also turns on the following optimization flags:
 -frerun-cse-after-loop  @gol
 -fsched-interblock  -fsched-spec @gol
 -fschedule-insns  -fschedule-insns2 @gol
+-fstore-merging @gol
 -fstrict-aliasing -fstrict-overflow @gol
 -ftree-builtin-call-dce @gol
 -ftree-switch-conversion -ftree-tail-merge @gol
@@ -8346,7 +8346,7 @@ early.  This flag is enabled by default at @option{-O} and higher.
 Perform merging of narrow stores to consecutive memory addresses.  This pass
 merges contiguous stores of immediate values narrower than a word into fewer
 wider stores to reduce the number of instructions.  This is enabled by default
-at @option{-O} and higher.
+at @option{-O2} and higher as well as @option{-Os}.
 
 @item -ftree-ter
 @opindex ftree-ter